docs: def encrypt_to_file(contents, filename):
      """
      Encrypts ``contents`` and writes it to ``filename``.
  
      ``contents`` should be a bytes string. ``filename`` should end with
      ``.enc``.
  
      Returns the secret key used for the encryption.
  
      Decrypt the file with :func:`doctr.travis.decrypt_file`.
  
      """
      if not filename.endswith('.enc'):
          raise ValueError("%s does not end with .enc" % filename)
  
      key = Fernet.generate_key()
      fer = Fernet(key)
  
      encrypted_file = fer.encrypt(contents)
  
      with open(filename, 'wb') as f:
          f.write(encrypted_file)
  
      return key